Fore vacuum pumps (or “primary vacuum pumps”) are defined as those which exhaust to atmospheric pressure. They are also required to support secondary pumps or to attain the initial conditions for their operation. There are two types of fore vacuum pumps:
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In this blog post, we'll examine the key working principles of common multi-stage roots fore vacuum pumps.
Multistage roots pumps are dry vacuum pumps used in low, medium, high and ultra-high vacuum systems to produce “dry” conditions.
The simple (single-stage) roots pump is most commonly employed as a booster pump for use in combination with several types of fore pumps (such as rotary vane pumps, screw and liquid ring pumps) to improve (or ‘boost’) ultimate pressure and pumping speeds. When multistage roots pumps are employed, no fore pump is required and they can operate from atmospheric pressure. A multi-stage roots pump can consist of up to eight stages and employ several sets of rotors (on a shared shaft). Roots pumps are suitable where a dry and clean atmosphere is important or, more likely, essential. Consequently, they are frequently used in the manufacture of semiconductors and solar panels, as well as for coatings, other industrial applications and for scientific instruments and research.
Building upon this simplistic model of a roots pump, a multi-stage roots pump (which can consist of up to eight stages) employs several sets of rotors (on a shared shaft). The geometry of the rotors creates compression and hence each stage produces a progressively higher pressure. In this way, the product of a lower stage is the “feed gas” for the next higher stage (but without any interconnecting valves).
As with single-stage (i.e. roots blower) pumps, in multistage units there is no contact between rotors and the stator housing — the units are hermetically sealed, they are robust and they can operate for long periods between service. Additionally, the new generation of multistage roots pumps have been specifically designed to be used in quiet and clean environments, such as in analytical instruments and research laboratories where — operating with pumping speeds of 25 to 200 m3/h — they create a very low noise of up to 52 dB(A).
The performance characteristics of roots pumps vary depending on a number of factors, including the number of stages, the unit size, rotation, speed of operation, ambient temperature, initial inlet pressure and the characteristics of the gases being pumped.
The following listings provide some of the more obvious and significant performance characteristics:
Multistage roots pumps are predominantly employed where their dry, clean and high pumping speed characteristics can be used to the best advantage. As a result their applications include: analytical instruments, R&D and space, the semiconductor and solar industries, and the laser industry. They’re also used in furnace or metallurgy and a variety of coating applications (where their very dry nature is highly advantageous).
Multistage roots pumps have many advantages over other pumps, and surprisingly few disadvantages or limitations.
The main advantages of multistage roots pumps are that they are very compact and quiet, enjoy a long service life, have no contact between moving parts and thus no wear, no particles generated, or any need for oil which would otherwise contaminate the vacuum system and final product.
However, multistage roots pumps have several disadvantages. These include: relatively high service costs and lower pumping performance when operating near atmospheric pressure. They are also only capable of achieving small pressure differences, which is why they are employed in a multistage format where each incremental stage is able to build upon the pressure increase achieved from the compression of the previous stage.
Advantages of Multistage Roots Pumps
Disadvantages of Multistage Roots Pumps
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The design principle of the Roots pumps was already invented in by Isaiah Davies, but it was 20 years later before it was implemented in practice by the Americans Francis and Philander Roots. Initially such pumps were used as blowers for combustion motors. Later, by inverting the drive arrangement, the principle was employed in gas meters. Only since has this principle been employed in vacuum engineering. Roots pumps are used in pump combinations together with backing pumps (rotary vane or dry pumps) and extend their operating range well into the medium vacuum range. With two stage Roots pumps this extends into the high vacuum range. The operating principle of Roots pumps permits the assembly of units having very high pumping speeds (over 100,000 m3/h) which often are more economical to operate than steam ejector pumps running in the same operating range.
A Roots vacuum pump (see Fig. 2.17) is a rotary positive-displacement type of pump where two symmetrically-shaped impellers rotate inside the pump casing past each other in close proximity. The two rotors have a cross section resembling approximately the shape of a figure 8 and are synchronized by a toothed gear. The clearance between the rotors and the casing wall as well as between the rotors themselves amounts only to a few tenths of a millimeter. For this reason, Roots pumps may be operated at high speeds without mechanical wear. In contrast to rotary vane and dry pumps, Roots pumps are not oil sealed, so that the internal leakage of dry compressing pumps by design results in the fact that compression ratios only in the range 10 – 100 can be attained. The internal leakage of Roots pumps, and also other dry compressing pumps for that matter, is mainly based on the fact that owing to the operating principle certain surface areas of the pump chamber are assigned to the intake side and the compression side of the pump in alternating fashion. During the compression phase these surface areas (rotors and casing) are loaded with gas (boundary layer); during the suction phase this gas is released. The thickness of the traveling gas layer depends on the clearance between the two rotors and between the rotors and the casing wall. Due to the relatively complex thermal conditions within the Roots pump it is not possible to base one’s consideration on the cold state. The smallest clearances and thus the lowest back flows are attained at operating pressures in the region of 1 mbar. Subsequently it is possible to attain in this region the highest compression ratios, but this pressure range is also most critical in view of contacts between the rotors and the casing.
The characteristic quantities to be found in equation 2.17 are only for the combination of the Roots pump and the backing pump, namely maximum compression k0 of the Roots pump and gradation kth between Roots pump and backing pump.
With the aid of the above equations the pumping speed curve of a given combination of Roots pump and backing pump may be calculated. For this the following must be known:
a) the theoretical pumping speed of the Roots pump: Sth
b) the max. compression as a function of the fore vacuum pressure: k0 (pV)
c) the pumping speed characteristic of the backing pump SV (pV)
The way in which the calculation is carried out can be seen in Table 2.3 giving the data for the combination of a Roots pump RUVAC WA / E 250 (single-stage rotary plunger pump, operated without gas ballast).
The method outlined above may also be applied to arrangements which consist of a rotary pump as the backing pump and several Roots pumps connected in series, for example. Initially one determines – in line with an iteration method – the pumping characteristic of the backing pump plus the first Roots pump and then considers this combination as the backing pump for the second Roots pump and so on. Of course, it is required that the theoretical pumping speed of all pumps of the arrangement be known and that the compression at zero throughput k0 as a function of the backing pressure is also known. As already stated, it depends on the vacuum process which grading will be most suitable. It may be an advantage when backing pump and Roots pump both have the same pumping speed in the rough vacuum range.
The amount of power drawn by the pump determines its temperature. If the temperature increases over a certain level, determined by the maximum permissible pressure difference pV – pa, the danger exists that the rotors may seize in the casing due to their thermal expansion. The maximum permissible pressure difference Δpmax is influenced by the following factors: forevacuum or compression pressure pV, pumping speed of the backing pump SV, speed of the Roots pump n, gradation kth and the adiabatic exponent κ of the pumped gas. Δpmax increases when pV and SV increase and decreases when n and kth increase. Thus the maximum difference between forevacuum pressure and intake pressure, pV-pa must - during continuous operation - not exceed a certain value depending on the type of pump. Such values are in the range between 130 and 50 mbar. However, the maximum permissible pressure difference for continuous operation may be exceed for brief periods. In the case of special designs, which use gas cooling, for example, high pressure differences are also permissible during continuous operation.
Standard flange-mounted motors are used as the drive. The shaft feedthroughs are sealed by two oil sealed radial shaft seals running on a wear resistant bushing in order to protect the drive shaft. Flange motors of any protection class, voltage or frequency may be used.
Integral leak tightness of this version is < 10-4 mbar · l · s-1.
In the case of better leak tightness requirements of < 10-5 mbar · l · s-1 the Roots pump is equipped with a canned motor. The rotor is seated in the vacuum on the drive shaft of the pump and is separated from the stator by a vacuum- tight non- magnetic tube. The stator coils are cooled by a fan having its own drive motor. Thus shaft seals which might be subject to wear are no longer required. The use of Roots pumps equipped with canned motors is especially recommended when pumping high purity-, toxic- or radioactive gases and vapors.
In the case of standard Roots pumps, measures must be introduced to ensure that the maximum permissible pressure difference between intake and exhaust port due to design constraints is not exceeded. This is done either by a pressure switch, which cuts the Roots pump in and out depending on the intake pressure, or by using a pressure difference or overflow valve in the bypass of the Roots pumps (Fig. 2.20 and 2.21). The use of an overflow valve in the bypass of the Roots pump is the better and more reliable solution. The weight and spring loaded valve is set to the maximum permissible pressure difference of the particular pump. This ensures that the Roots pump is not overloaded and that it may be operated in any pressure range. In practice this means that the Roots pump can be switched on, together with the backing pump, at atmospheric pressure. In the process any pressure increases will not adversely affect combined operation, i.e. the Roots pump is not switched off in such circumstances.
In the case of Roots pumps with pre-admission cooling (Fig. 2.22), the compression process basically is the same as that of a normal Roots pump. Since greater pressure differences are allowed more installed power is needed, which at the given speed and the pressure difference between inlet and discharge port is directly proportional and is composed of the theoretical work done on compression and various power losses. The compression process ends normally after opening of the pumping chamber in the direction of the discharge port. At this moment warmed gas at higher pressure flows into the pumping chamber and compresses the transported volume of gas. This compression process is performed in advance in the case of pre-admission cooling. Before the rotor opens the pumping chamber in the direction of the discharge port, compressed and cooled gas flows into the pumping chamber via the pre-admission channel. Finally, the rotors eject the pumped medium via the discharge port. The cooled gas, which in the case of single- stage compression is taken from the atmosphere and admitted from the pre-admission cooler, and which in the case of multi-stage pump systems is taken from downstream gas coolers, performs a pre-compression and removes by “inner cooling” the heat of compression at the point of time it occurs.
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